1. Field of the Invention
The present invention relates to an electrophotographic photoconductor, and more particularly to an electrophotographic photoconductor which is useful for copiers, facsimile machines, laser printers, digital printing plate manufacturing apparatus and the like.
2. Discussion of the Related Art
Electrophotographic recording methods using a photoconductor are widely used for copiers, facsimile machines, laser printers, digital printing plate manufacturing apparatus and the like. The methods include, for example, the following processes:
(1) charging a photoconductor; PA1 (2) imagewise irradiating the photoconductor with light to form an electrostatic latent image; PA1 (3) developing the latent image with a toner to form a toner image on the photoconductor; PA1 (4) transferring the toner image onto an image receiving material such as receiving paper; PA1 (5) fixing the toner image on the receiving material to form a fixed toner image; and PA1 (6) cleaning the photoconductor to perform the next image forming processes. PA1 (1) having a good charging property so as to be charged to an appropriate electric potential in a dark place; PA1 (2) having a good charge maintaining property such that the decrease of the electric potential is little in a dark place; and PA1 (3) having a good charge dissipating property such that the electric potential is rapidly dissipated by light irradiation. PA1 (4) having a relatively low cost; PA1 (5) hardly causing environmental pollution; and PA1 (6) producing good images without image defects such as background fouling for a long time. PA1 (1) selenium photoconductive layers including selenium or a selenium alloy as a main component; PA1 (2) inorganic photoconductive layers which include an inorganic photoconductive material such as zinc oxide or cadmium sulfide which is dispersed in a binder resin; PA1 (3) amorphous silicon photoconductive layers which include an amorphous silicon material; and PA1 (4) organic photoconductive layers which include an organic photoconductive material. PA1 (1) photoconductive resin type photoconductors which include a photoconductive resin such as polyvinyl carbazole; PA1 (2) charge transfer complex type photoconductors which include a charge transfer complex such as polyvinyl carbazole-trinitrofluorenone; PA1 (3) pigment dispersion type photoconductors which include an organic pigment such as phthalocyanine which is dispersed in a binder resin; and PA1 (4) functionally separated photoconductors which include a combination of a charge generating material and a charge transporting material. PA1 (1) when light irradiates a charged organic photoconductor, the light passes through a transparent charge transporting layer and is absorbed by a charge generating material included in a charge generating layer; PA1 (2) the charge generating material which has absorbed the light generates a charge carrier; PA1 (3) the charge carrier, which is injected to the charge transporting layer, moves through the charge transporting layer, which is caused by the electric field formed in the charged photoconductor; and PA1 (4) the charge carrier finally combines with the charge on the surface of the photoconductor, resulting in neutralization of the charge, and thereby an electrostatic latent image is formed.
The requisites for electrophotographic photoconductors are, for example, as follows:
Currently, in addition to these requisites, electrophotographic photoconductors are especially required to have the following requisites:
Conventionally, photoconductors including the following photoconductive layers are well known as an electrophotographic photoconductor:
Among these photoconductors, photoconductors having an organic photoconductive layer are widely used because they have a relatively low cost, various types of photoconductors can be designed and they hardly cause environmental pollution.
Organic photoconductors are broadly classified as follows:
Currently, among these organic photoconductors, functionally separated photoconductors attract considerable attention.
The mechanism of formation of an electrostatic latent image is as follows:
Functionally separated photoconductors which include a combination of a charge transporting material which has absorbance mainly in an ultraviolet region and a charge generating material which has absorbance mainly in a visible region are well known and preferable. However, even in the functionally separated photoconductors, the durability is not necessarily satisfactory. As mentioned above, the electrophotographic photoconductors are recently required to have good durability. Therefore, it is very important for the electrophotographic photoconductors to continue to produce good images for a long period of time.
In order to continue to produce good images for a long period of time, it is essential to obtain techniques to prevent occurrence of image defects such as background fouling, and to prevent decrease of image density, even when used for a long time. It is well known that the image defects and the decrease of image density are respectively caused by faults on the surface of the photoconductors, and decrease of the electric potential or increase of the residual potential of the photoconductors after the light irradiation. However, an electrophotographic photoconductor, which has both of good abrasion resistance and good durability in charge properties, has not been developed, and it is especially desired.
In attempting to improve the abrasion resistance and the durability, various proposals have been made.
At first, the proposals which have been made to improve the abrasion resistance of the surface of the photoconductors are as follows:
(1) Abrasion Resistance Improving Methods by Improving Mechanical Strength of Charge Transporting Layer
For example, Japanese Laid-Open Patent Publications Nos. 10-288846 and 10-239870 have disclosed photoconductors in which the abrasion resistance thereof is improved by using a polyacrylate resin as a binder resin.
Japanese Laid-Open Patent Publications Nos. 9-160264 and 10-239871 have disclosed photoconductors in which the abrasion resistance thereof is improved by using a polycarbonate resin as a binder resin.
Japanese Laid-Open Patent Publications Nos. 10-186688, 10-186687, and 5-040358 have disclosed photoconductors in which the abrasion resistance thereof is improved by using a polyester resin having a terphenyl skeleton, a polyester resin having a triphenyl methane skeleton, or a polyester resin having a fluorene skeleton as a binder resin.
(2) Abrasion Resistance Improving Methods by Decreasing Friction Coefficient of Charge Transporting Layer
For example, Japanese Laid-Open Patent Publications Nos. 10-246978 and 10-20534 have disclosed photoconductors which have a relatively low friction coefficient by including a siloxane component. Japanese Laid-Open Patent Publications Nos. 5-265241 and 8-328286 have disclosed photoconductors which have a relatively low friction coefficient by including a particulate fluorine containing resin.
(3) Abrasion Resistance Improving Methods by Reinforcing Charge Transporting Layer
For example, Japanese Laid-Open Patent Publications Nos. 1-129260 and 8-101517 have disclosed photoconductors in which the abrasion resistance thereof is improved by including a filler in a charge transporting layer.
Japanese Laid-Open Patent Publications Nos. 9-12637 and 9-235442 have disclosed photoconductors in which the abrasion resistance thereof is improved by using a polymer blend including a styrene elastomer as a binder resin in a charge transporting layer.
The photoconductors mentioned in (1) to (3) have to include a large amount of a charge transporting material having low molecular weight in the photoconductive layer because of obtaining a good light decaying property, i.e., good photosensitivity. To use a large amount of a charge transporting material having low molecular weight seriously deteriorates the strength of the photoconductive layer, and the more the charge transporting material is included in the photoconductive layer, the worse the abrasion resistance of the photoconductive layer. Therefore the photoconductive layers of these photoconductors easily abrade, which is caused by the charge transporting material having low molecular weight. Accordingly the methods mentioned above are not effective for the improvement of abrasion resistance of photoconductors.
Other methods, which have been disclosed to improve the abrasion resistance of the surface of the photoconductors, are as follows:
(4) Abrasion Resistance Improving Method by Providing Protective Layer
For example, Japanese Laid-Open Patent Publication No. 10-177268 discloses a photoconductor in which the abrasion resistance thereof is improved by providing a protective layer formed on a charge transporting layer.
However, when a protective layer is formed, an oxidizing material tends to stay on the surface of the photoconductor, resulting in sometimes occurrence of image defects such as image tailing. In addition, the sensitivity of the photoconductor tends to deteriorate, and therefore this method is not effective for the improvement of the abrasion resistance.
(5) Abrasion Resistance Improving Method Using Charge Transporting Polymer Material
Japanese Laid-Open Patent Publication No. 7-325409 discloses a photoconductor which includes a charge transporting polymer material instead of charge transporting materials having low molecular weight. It is supposed that the photoconductor has good abrasion resistance because the content of resins in the photoconductive layer is relatively high. However, when the charge transporting polymer material is used in such an amount that the photoconductor has good abrasion resistance, another problem such as background fouling occurs. Thus, the photoconductor including a charge transporting polymer material cannot improve its abrasion resistance while stably producing images having good image qualities.
As mentioned above, there is no photoconductor which has good abrasion resistance and can stably produce good images.
On the other hand, proposals which have been made to improve the stability of the image qualities of images produced by photoconductors are as follows:
(6) Image Stability Improving Methods Using Antioxidant
For example, Japanese Laid-Open Patent Publications Nos. 57-122444 and 61-156052 have disclosed photoconductors which include an antioxidant in the photoconductive layer.
(7) Image Stability Improving Methods Using Plasticizer
For example, Japanese Laid-Open Patent Publications Nos. 8-272126 and 8-95278 have disclosed photoconductors which include a plasticizer in the photoconductive layer.
The methods mentioned in (6) and (7) are effective for the prevention of deterioration of the charge properties of the photoconductive layer when the photoconductor is used for a long time. When these compounds are used for a photoconductor which includes a binder resin and a charge transporting material having low molecular weight, since the charge transporting material is included therein in a large amount, only a small amount of these compounds can be added. Therefore, these methods are not effective for the improvement of the durability of the photoconductor. In addition, the charge transporting layer, which includes a charge transporting material, generally has a relatively low glass transition temperature, and when these compounds are added therein, the glass transition temperature decreases to a temperature which is almost the same as the inside temperature of an image forming apparatus in which the photoconductor is provided. Therefore, other problems such as deformation of the photoconductive layer and toner adhesion to the photoconductive layer tend to occur. Therefore, these methods are also not effective for the improvement of the durability of the photoconductor.
Therefore, a photoconductor which can produce images having good image qualities for a long period of time cannot be obtained by the techniques which have been conventionally proposed.